Halaman

Kamis, 15 Desember 2011

A Circus Tent for Fukushima Daiichi?

Tokyo Electric Power Co.

The cover to block the spread of radiation from Fukushima Daiichi’s No. 1 reactor under construction on Sept. 17.

The polyester cover erected over Unit 1 of Japan’s stricken Fukushima Daiichi nuclear power station was fashioned in the shape of a tight-fitting, non-descript, white box.
It could have looked very different.

At first, planners from plant operator Tokyo Electric Power Co. and general contractor Shimizu Corp. say they considered something that looked like a circus tent, when they sat down in mid-March to discuss how to stop the spread of radioactive materials from the plant’s four damaged reactor buildings.

Constructing a tent was fast, and Tepco was in a hurry to stop the spread of radiation – particularly from Units 1 and 3, where hydrogen explosions after the nuclear accident in March had blown away the reactor buildings. The engineers finally decided the boxy shape, with panels fastened to a steel frame, would be more durable.

“We dropped the idea [of putting up a plain tent over the plant] because it would be easily blown away by winds in a typhoon,” explained Noriyoshi Nakamura, the plant’s general manager responsible for radiation containment. “It wouldn’t cover the reactor as tightly as we wanted, either.”
Shimizu’s general manager for construction technology agrees. “

The cover we built is expected to last at least a few years, but in the future, we probably need a more robust structure,” said Masahiro Indo. “The cover isn’t exactly air-tight; air can leak through gaps between the panels.”
The Wall Street Journal describes some of the civil-engineering heroics Shimizu went through to get the cover in place.

But even before the construction began, there were challenges. There was no detailed plant blueprint, for one. That information was trapped in a computer in the reactor building and inaccessible. The only layout available was one produced 40 years ago, which didn’t include any of the stacks, pipes and buildings added later.

Shimizu ended up doing a laser scan of the entire building and creating a 3-D image, then building a 1/100-scale model of Unit 1, complete with surrounding debris. “We rebuilt the model three or four times until we were satisfied with it,” Mr. Indo said.

The cover is made up of 20-meter-square polyester sheets, with each sheet held down by two weights weighing 7 and 12 tons. The panels were lifted by two cranes capable of carrying 750-ton loads, out of only 14 such machines in all of Japan.

During the construction, those cranes got dosed with high levels of radiation, particularly the one working closest to Unit 1. So where are the contaminated cranes now? Tepco says they’re still sitting in the plant complex, waiting for their next mission.